fuel consumption

i have a 2002 FL 1 TD4 Auto i get around 25-26 around town and have got up to 37mpg on a run, soon as i go 80mph it will go down to about 20mpg so i let it sit on 70mph and then it goes up to about 35mpg

to say the least im very happy with that considering its shaped like a house brick and weighs going on 2 ton.

who makes the FL 2 2.2 engine is it a bmw engine, is it alot better would you say than the TD4 engine.

thanks

gary